The present invention relates to a novel process for the preparation of vinyl chloride-based resins or, more particularly, to a process for the preparation of vinyl chloride-based resins modified with a polymer latex by the suspension polymerization of vinyl chloride monomer or a monomer mixture mainly composed of vinyl chloride in an aqueous medium.
As is well know, polyvinyl chloride resins are prepared mostly by the suspension polymerization of the monomer or monomers in an aqueous medium in the presence of a suspending agent and a polymerization initiator soluble in the monomer phase. The polyvinyl chloride resins thus obtained are fabricated into various kinds of shaped articles to be used in a wide variety of applications. It is a usual practice that, when improvement in some properties of the resin powder per se or the shaped articles thereof is desired, the resin powder is admixed, prior to fabrication, uniformly with various kinds of additives according to the object including impact strength improvers, heat stabilizers, workability improvers and the like.
For example, impact strength in the shaped articles of vinyl chloride-based resins can be improved by blending the resin with various kinds of polymers such as copolymeric resins of acrylonitrile, butadiene and styrene (ABS resins), copolymeric resins of methyl methacrylate, butadiene and styrene (MBS resins), copolymeric resins of ethylene and vinyl acetate (EVA resins) and the like and the workability of the resin powder can be improved with methacrylic ester-based resins and styrene-based resins.
It is of course desirable that, in order that improvement of the objective property of the resin powder or the shaped articles thereof can be achieved as far as possible by the admixture of the above mentioned modifier resins, the admixing of the vinyl chloride-based resin and the modifier polymer is as intimate as possible microscopically so that the modifier polymer should be in a form of very finely divided powder.
Whereas the finely divided powder of the above mentioned modifier polymers for the improvement of impact strength of the shaped articles or the workability of the polyvinyl chloride resin is usually obtained by drying and pulverizing the latex of the respective polymers prepared by the emulsion polymerization of the monomers in an aqueous medium, handling of such powders is not always performed with ease and convenience and uniform blending of them with the polyvinyl chloride resin is carried out only with considerable difficulty. Consequently, the improvement of the desired properties is also sometimes far from satisfaction by the dry blending described above.
Accordingly, an improved method for the admixture of the modifier polymer with the polyvinyl chloride resin has been proposed in which the modifier polymer is admixed as an emulsion or latex thereof into the aqueous polymerizate slurry of the polyvinyl chloride resin after completion of the polymerization reaction whereby the particles of the modifier polymer are absorbed or adsorbed on the resin particles of the polyvinyl chloride resin and the aqueous slurry of the polyvinyl chloride polymerizate having the modifier polymer particles adsorbed thereon is dehydrated and dried to give the finished product. This method is, however, defective in two aspects. Firstly, the adsorption of the particles of the latex polymer does not always take place completely so that uniformity in the blending of the modifier polymer with the polyvinyl chloride resin cannot be satisfactory at least microscopically. Secondly, the waste water discharged from the dehydration procedure contains considerable amount of the latex polymer suspended therein to render the water turbid so that the discharging of the waste water into the environmental hydrosphere causes a serious problem of pollution which can be solved only with great economical disadvantages.
To eliminate the above disadvantages in the process of the addition of the modifier polymer latex into the aqueous polymerizate slurry of the polyvinyl chloride resin after completion of the polymerization reaction, a further improved method has been proposed in which the latex of the modifier polymer is added into the aqueous polymerization medium for the suspension polymerization of vinyl chloride and the polymerization is carried out in the presence of the latex of the modifier polymer. This method is also not free from several disadvantages that the addition of the latex adversely affects the condition of dispersion of the monomer droplets in the aqueous medium leading to the formation of coarser particles of the polyvinyl chloride resin and the polymer scale deposition on the walls of the polymerization reactor is markedly increased.
Furthermore, the adsorption of the latex particles on the polyvinyl chloride resin particles is also not complete even in this method so that the problem of the environmental pollution of the hydrosphere by the waste water discharged in the dehydration of the polymerizate slurry cannot be solved to a satisfactory extent along with the increased cost due to the loss of considerable amount of the modifier polymer in the waste water.